5.9 Continuous Reactions

All of the chemical reactions discussed in this chapter have been discontinuous reactions with a fixed reaction stoichimetry and a reaction line or curve on a T-P stability diagram. When one or more of the phases involved in a chemical reaction has a variable composition, such as a magma or a solid solution mineral, the chemical reaction may be a continuous reaction. Continuous reactions have a changing reaction stoichimetry and occur over a range of temperatures at a chosen pressure. During a continuous reaction, at least one of the reactant or product phases changes composition.

A familiar example of a continuous reaction is the heating of a saturated solution of salt (brine) or sugar (syrup). If a sugar-saturated syrup (undissolved sucrose crystals are present), the equilibrium syrup composition will become more sucrose-rich, continuously, as the temperature rises. You can see this in Figure 5.22 on a graph of experiments on the saturated syrup composition (weight percent sucrose) as a function of temperature. Click on the diagram to see a larger version, then click the "Coordinates" button and move your mouse along the sucrose-saturation curve to see numerically how the expected syrup composition changes with increasing temperature.

Because all liquids may have variable chemical compositions, most reactions involving magmas and minerals such as melting/crystallization reactions are continuous reactions. You can see this on other temperature-composition phase diagrams such as the diopside-anorthite T-X diagram and read more about it in the chapter on Saturation Diagrams. Although there commonly are special temperatures and associated liquid compositions for which the melting/crystallization reaction is discontinuous, these typically involve an additional phase (e.g. a eutectic reaction) or a reduced number of components (e.g. ice alone melting). An example of a discontinuous melting/crystallization reaction is the ice-syrup-sucrose eutectic at -13.9°C visible on Figure 5.22.

For continuous reactions, the same phase may be both a reactant and a product, but with slightly different chemical compositions. When a sucrose-saturated syrup is heated, the reaction is:

sucrose + syrup (X) = syrup (X+)

where the product syrup (X+) has a higher weight percent sucrose than the reactant syrup (X). If the mineral melting is a solid solution, like plagioclase, then the mineral is both a reactant and product. The plagioclase melting reaction is:

plagioclase (An_X) + magma (An_Y) = plagioclase (An_X+) + magma (An_Y+)

where "An_X" is the Anorthite component of the plagioclase and "An_Y" is the Anorthite component of the liquid. As the temperature rises, at equilibrium both the plagioclase and the magma become An-richer, but the proportion of plagioclase decreases and the proportion of melt increases. You can see this on the plagioclase melting diagram in Figure 5.23. Click on the diagram to see a larger version, then click on he "Show Phase %" button and mouseover the diagram to see nmerically how the compositions of the plagioclase and the melt change as temperature is increased. You can read more about this in the chapter on Binary Solid Solution Diagrams.



Next Page